In a typical digital signal processing technique an analog signal such as, an audio signal, an image signal, or the like, is converted to a digital signal by means of a sampling operation and a quantizing operation, and then processed by a digital filter, such as a finite impulse response (FIR) filter (a transversal filter or a non-recursive filter) which is well known in the art. An output of the digital filter is transferred to a digital-to-analog converter, which it is converted into an analog signal. The converted analog signal is taken out through a low pass filter, or the like.
A rate conversion is required to improve the signal-to-noise ratio or to decrease the burden on the low pass filter. This conversion is performed by, for example, interpolating new digital data between adjacent sampled digital data using the digital filter. For instance, if digital data is interpolated between consecutive samples of digital data, the number of sampler is doubled, and a two-times rate-conversion is achieved. However, a conventional digital filter having the rate-conversion function has a defect that high-speed filtering can not be performed since the interpolating operation is required.
Further, in order to process a plurality of digital signals, digital filters equal in number to the signals to be processed must be employed. Thus, a large quantity of hardware elements are required for digital-filtering. Therefore, a plurality of semiconductor chips, in each of which a digital filter circuit is independently formed, must be used.
A transversal (non-recursive) digital filter generally comprises a plurality of delay circuits, a plurality of multipliers and an adder. When this filter is produced as a semiconductor device, a read-write memory (RAM) is used as the delay circuits. Further, a read only memory (ROM) is used to store a plurality of coefficients for multiplications. In addition to these memories, a plurality of multiplier circuits and an adder circuit are used. Therefore, it is very hard to integrate a plurality of filters in a semiconductor chip.
An object of the present invention is to provide a digital signal processing apparatus having a digital filter in which the rate-conversion can be performed at a high speed.
Another object of the present invention is to provide a digital signal processing apparatus adaptable for filtering a plurality of digital signals (multi-channel signals) with a small number of hardware elements.
Still other object of the present invention is to provide a digital signal processing apparatus having a digital filter means formed on a semiconductor chip and performing a rate-conversion operation for at least two different digital signals in a multi-channel digital system.